The newly identified fungal pathogen, Batrachochytrium, has recently been implicated as causing the decline of amphibians worldwide. We aim to determine the origin of chytridiomycosis (the disease caused by Batrachochytrium), and understand how anthropogenic environmental changes influence its prevalence and spread. Our research will focus specifically on the relationship between Batrachochytrium and the mountain yellow-legged frog (Rana muscosa), a once-common amphibian in California's Sierra Nevada mountains. R. muscosa is an ideal study host as it is still common enough despite population declines to be used in experiments, and populations are strongly influenced by chytridiomycosis infection. As with animal populations throughout the world, amphibians in the Sierra Nevada are being subjected to an increasing array of anthropogenic stressors including exotic species introductions, habitat fragmentation, and chemical contaminants and these may change the prevalence and spread of disease. Using a team with expertise in pathogenic fungi, epidemiology, amphibian ecology, population genetics, and statistical and mechanistic modeling, our specific objectives are to: 1) describe the origin of Batrachochytrium to determine whether its association with amphibians in the Sierra Nevada represents an old association or a recent expansion of the fungus into previously uninfected frogs; 2) describe the effect of infection and spread on frog population dynamics using temporal resurveys of sites identified from an existing database of 4,500 water bodies in the Sierra Nevada; 3) develop statistical models to describe the effect of nonnative fish, habitat fragmentation, and windborne agricultural contaminants on the distribution of frogs and chytridiomycosis; and 4) develop detailed mechanistic models to predict the likely effects of various biological mechanisms (both natural and anthropogenic) on the incidence, spatial distribution, and persistence of chytridiomycosis, and to predict the impact of this disease on the abundance and dynamics of R. muscosa. Given the paucity of studies of disease in animal populations in the wild, the results of our study will provide important insights into disease dynamics in other systems, and aid in efforts to protect biodiversity in the face of emerging infectious diseases.